In the past food products have been packaged in injection molded plastic containers. These containers are manufactured at an injection molding plant and shipped empty to a food processing facility. There the containers are filled and a flexible sheet is sealed in place to close their tops. The filled containers are then packaged and shipped to restuarants or retail food outlets.
At the injection molding plant empty containers for food products must be conveniently stackable in a nested condition. The draft angle required to ease an injection molded container from the mold members in which it is formed contributes as well to easy stacking of the finished container. It is known to provide spacers which project outwardly from containers to keep a predetermined minimum amount of space between nested containers. Spacers which are stable can support a large stack of containers, and thus can be shipped and handled more economically.
Food containers have been provided with flanges at their open ends, and a flexible sheet material has been welded or heat sealed across the open end for the usual hygenic reasons. When the food within is to be served or eaten, the sheet may be either cut away or peeled from the top of the container, depending on how it is welded or heat sealed into place.
There are two techniques conventionally used to seal a sheet material to the top of a container. In one process, the sheet is streched across the top of the container including the surrounding flange. Then a hot ring (or rings) is pressed against the sheet and the heat welds the sheet to the flange. In this case the ring is relatively narrow compared to the width of the flange so one or more concentric circles of welded or heat fused areas are created. Generally, sheets attached in this manner must be cut to gain access to the food within because these welds cannot be peeled apart.
The second type of sealing process is similar except that a lower heat is applied over a wider area, typically the entire surface of the flange. When the sheet is sealed in place with this technique, it generally can be peeled off the flange to open the container. Regardless of which welding technique is used, occassionally the seal is not sufficient. When this occurs the container and the food within must be discarded as waste. Therefore, any steps which promote good seals with a minimum of welding time would advance the art.
Once a container has been filled with a product and its top sheet sealed in place, the container is placed four to a square pallet or box, and the pallets are stacked for shipment. The pallets or boxes are frequently stacked one on top of another and then packed on a truck or railroad car using a forklift. Containers which can withstand greater compressive loads without collapsing can be stacked higher and will withstand rougher handling.